how to bake high poly and texture normal map together
See How does normal baking work? on how normal maps are created.

You can't bake other textures and the normal into a normal map because the normal map encodes the direction of the normals in the RGB channels of the image representing the map.

From Bump_and_Normal  (Linked to 2.4 because there is more info)


  Normal maps in Blender store a normal as follows:
  
  
  Red maps from (0-255) to X (-1.0 - 1.0)
  Green maps from (0-255) to Y (-1.0 - 1.0)
  Blue maps from (0-255) to Z (0.0 - 1.0)
  


Since can add more than one texture to a model this isn't a restriction. 

EDIT: A normal vector is perpendicular to a face its coordinates (x,y,z) are stored as a pixel in an image map.